Polypropylene ( PP ) Plastics : Types, Uses, Properties, Application & Structure info................S.S
What is Polypropylene (PP), and What is it Used For?
Polypropylene (PP) is a thermoplastic “addition polymer” made from the combination of propylene monomers. It is used in a variety of applications to include packaging for consumer products, plastic parts for various industries including the automotive industry, special devices like living hinges, and textiles. Polypropylene was first polymerized in 1951 by a pair of Phillips petroleum scientists named Paul Hogan and Robert Banks and later by Italian and German scientists Natta and Rehn. It became prominent extremely fast, as commercial production began barely three years after Italian chemist, Professor Giulio Natta, first polymerized it. Natta perfected and synthesized the first polypropylene resin in Spain in 1954, and the ability of polypropylene to crystallize created a lot of excitement. By 1957, its popularity had exploded and widespread commercial production began across Europe. Today it is one of the most commonly produced plastics in the world.
CNC Cut Polypropylene Living Hinge Prototype Child Safe Lid by Creative Mechanisms
According to some reports, the current global demand for the material generates an annual market of about 45 million metric tons and it is estimated that the demand will rise to approximately 62 million metric tons by 2020. The major end users of polypropylene are the packaging industry, which consumes about 30% of the total, followed by the electrical and equipment manufacturing, which uses about 13% each. Household appliances and automotive industries both consume 10% each and construction materials follows with 5% of the market. Other applications together make up the rest of the global polypropylene consumption.
Polypropylene has a relatively slippery surface which can make it a possible substitute for plastics like Acetal (POM) in low friction applications like gears or for use as a contact point for furniture. Perhaps a negative aspect of this quality is that it can be difficult to bond Polypropylene to other surfaces (i.e. it does not adhere well to certain glues that work fine with other plastics and sometimes has to be welded in the event that forming a joint is required). Although polypropylene is slippery at the molecular level, it does have a relatively high coefficient of friction - which is why acetal, nylon, or PTFE would be used instead. Polypropylene also has a low density relative to other common plastics which translates to weight savings for manufacturers and distributors of injection molded Polypropylene parts. It has exceptional resistance at room temperature to organic solvents like fats but is subject to oxidation at higher temperatures (a potential issue during injection molding).
One of the major benefits of Polypropylene is that it can be manufactured (either through CNC or injection molding, thermoforming, or crimping) into a living hinge. Living hinges are extremely thin pieces of plastic that bend without breaking (even over extreme ranges of motion nearing 360 degrees). They are not particularly useful for structural applications like holding up a heavy door but are exceptionally useful for non load-bearing applications such as the lid on a bottle of ketchup or shampoo. Polypropylene is uniquely adept for living hinges because it does not break when repeatedly bent. One of the other advantages is that polypropylene can be CNC machined to include a living hinge which allows for faster prototype development and is less expensive than other prototyping methods. Creative Mechanisms is unique in our ability to machine living hinges from a single piece of polypropylene.
Another advantage of Polypropylene is that it can be easily copolymerized (essentially combined into a composite plastic) with other polymers like polyethylene. Copolymerization changes the material properties significantly, allowing for more robust engineering applications than are possible with pure polypropylene (more of a commodity plastic on its own).
The characteristics mentioned above and below mean that polypropylene is used in a variety of applications: dishwasher safe plates, trays, cups, etc, opaque to-go containers, and many toys.
What are the Characteristics of Polypropylene?
Some of the most significant properties of polypropylene are:
- Chemical Resistance: Diluted bases and acids don’t react readily with polypropylene, which makes it a good choice for containers of such liquids, such as cleaning agents, first-aid products, and more.
- Elasticity and Toughness: Polypropylene will act with elasticity over a certain range of deflection (like all materials), but it will also experience plastic deformation early on in the deformation process, so it is generally considered a "tough" material. Toughness is an engineering term which is defined as a material's ability to deform (plastically, not elastically) without breaking..
- Fatigue Resistance: Polypropylene retains its shape after a lot of torsion, bending, and/or flexing. This property is especially valuable for making living hinges.
- Insulation: polypropylene has a very high resistance to electricity and is very useful for electronic components.
- Transmissivity: Although Polypropylene can be made transparent, it is normally produced to be naturally opaque in color. Polypropylene can be used for applications where some transfer of light is important or where it is of aesthetic value. If high transmissivity is desired then plastics like Acrylic or Polycarbonate are better choices.
Polypropylene is classified as a “thermoplastic” (as opposed to “thermoset”) material which has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (roughly 130 degrees Celsius in the case of polypropylene). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polypropylene liquefy, which allows them to be easily injection molded and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.
Why is Polypropylene used so often?
Polypropylene is used in both household and industrial applications. Its unique properties and ability to adapt to various fabrication techniques make it stand out as an invaluable material for a wide range of uses. Another invaluable characteristic is polypropylene’s ability to function as both a plastic material and as a fiber (like those promotional tote bags that are given away at events, races, etc). Polypropylene’s unique ability to be manufactured through different methods and into different applications meant it soon started to challenge many of the old alternative materials, notably in the packaging, fiber, and injection molding industries. Its growth has been sustained over the years and it remains a major player in the plastic industry worldwide.
At Creative Mechanisms, we have used polypropylene in a number of applications across a range of industries. Perhaps the most interesting example includes our ability to CNC machine polypropylene to include a living hinge for prototype living hinge development. Polypropylene is a very flexible, soft material with a relatively low melting point. These factors have prevented most people from being able to properly machine the material. It gums up. It doesn’t cut clean. It starts to melt from the heat of the CNC cutter. It typically needs to be scraped smooth to get anything close to a finished surface. But we have been able to solve this problem which allows us to create novel prototype living hinges out of polypropylene. Take a look at the video below:
What Are The Different Types of Polypropylene?
There are two main types of polypropylene available: homopolymers and copolymers. The copolymers are further divided into block copolymers and random copolymers. Each category fits certain applications better than the others. Polypropylene is often called the “steel” of the plastic industry because of the various ways in which it can be modified or customized to best serve a particular purpose. This is usually achieved by introducing special additives to it or by manufacturing it in a very particular way. This adaptability is a vital property.
Homopolymer polypropylene is a general-purpose grade. You can think of this like the default state of the polypropylene material. Block copolymer polypropylene has co-monomer units arranged in blocks (that is, in a regular pattern) and contain anywhere between 5% to 15% ethylene. Ethylene improves certain properties, like impact resistance while other additives enhance other properties. Random copolymer polypropylene – as opposed to block copolymer polypropylene – has the co-monomer units arranged in irregular or random patterns along the polypropylene molecule. They are usually incorporated with anywhere between 1% to 7% ethylene and are selected for applications where a more malleable, clearer product is desired.
How is Polypropylene made?
Polypropylene, like other plastics, typically starts with the distillation of hydrocarbon fuels into lighter groups called “fractions” some of which are combined with other catalysts to produce plastics (typically via polymerization or polycondensation).
Polypropylene for Prototype Development on CNC Machines, 3D Printers, & Injection Molding Machines:
3D Printing Polypropylene:
Polypropylene is not readily available in filament form for 3D printing.
CNC Machining Polypropylene:
Polypropylene is widely used as sheet stock for CNC machine manufacturing. When we prototype a small number of polypropylene parts we typically CNC machine them. Polypropylene has gained a reputation as a material that cannot be machined. This is because it has a low annealing temperature, which means that it starts to deform under heat. Because it is a very soft material in general, it requires an extremely high skill level to be cut with precision. Creative Mechanisms has been successful in doing so. Our teams can use a CNC machine and cut the polypropylene cleanly and with extremely great detail. In addition, we are able to create living hinges with polypropylene that have thickness as little as .010 inches. Making living hinges is a difficult endeavor on its own, which makes using a difficult material like polypropylene even more impressive.
Injection Molding Polypropylene:
Polypropylene is a very useful plastic for injection molding and is typically available for this purpose in the form of pellets. Polypropylene is easy to mold despite its semi-crystalline nature, and it flows very well because of its low melt viscosity. This property significantly enhances the rate at which you can fill up a mold with the material. Shrinkage in polypropylene is about 1-2% but can vary based on a number of factors, including holding pressure, holding time, melt temperature, mold wall thickness, mold temperature, and the percentage and type of additives.
Other:
In addition to the conventional plastic applications, polypropylene also lends itself well to fiber applications. This gives it an even wider range of uses that go beyond just injection molding. Those include ropes, carpets, upholstery, clothing, and the like.
Image From AnimatedKnots.com
What are the Advantages of Polypropylene?
- Polypropylene is readily available and relatively inexpensive.
- Polypropylene has high flexural strength due to its semi-crystalline nature.
- Polypropylene has a relatively slippery surface.
- Polypropylene is very resistant to absorbing moisture.
- Polypropylene has good chemical resistance over a wide range of bases and acids.
- Polypropylene possesses good fatigue resistance.
- Polypropylene has good impact strength.
- Polypropylene is a good electrical insulator.
What are the Disadvantages of Polypropylene?
- Polypropylene has a high thermal expansion coefficient which limits its high temperature applications.
- Polypropylene is susceptible to UV degradation.
- Polypropylene has poor resistance to chlorinated solvents and aromatics.
- Polypropylene is known to be difficult to paint as it has poor bonding properties.
- Polypropylene is highly flammable.
- Polypropylene is susceptible to oxidation.
Despite its shortcomings, polypropylene is a great material overall. It has a unique blend of qualities that aren’t found in any other material which makes it an ideal choice for many projects.
What are the properties of Polypropylene?
Property | Value |
Technical Name | Polypropylene (PP) |
Chemical Formula |
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Resin Identification Code (Used For Recycling) |
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Melt Te mperature | 130°C (266°F) |
Typical Injection Mold Temperature | 32 - 66 °C (90 - 150 °F) *** |
Heat Deflection Temperature (HDT) | 100 °C (212 °F) at 0.46 MPa (66 PSI) ** |
Tensile Strength | 32 MPa (4700 PSI) *** |
Flexural Strength | 41 MPa (6000 PSI) *** |
Specific Gravity | 0.91 |
Shrink Rate | 1.5 - 2.0 % (.015 - .02 in/in) *** |
(C3H6)n
What is Polypropylene and What It's h//twitter.com/SunnyRa24637740?s=0 for?
Polypropylene is a tough, rigid and crystalline thermoplastic produced from propene (or propylene) monomer. It is a linear hydrocarbon resin. The chemical formula of polypropylene is (C3H6)n. PP is among the cheapest plastics available today.
PP belongs to polyolefin family of polymers and is one of the top three widely used polymers today. Polypropylene has applications both as a plastic and a fiber in:
- Automotive Industry
- Industrial Applications
- Consumer Goods, and
- Furniture Market
It has the lowest density among commodity plastics.
Some of the key suppliers of polypropylene are:
- A. Schulman - GAPEX®, ACCUTECH™, POLYFORT®, Fiberfil®, FERREX® and more
- Borealis - Daplen™, Bormed™, Fibremod™ and more
- ExxonMobil Chemical - ExxonMobil™, Achieve™
- LyondellBasell - Adstif, Circulen, Hifax, Hostacom, Moplen and more
- SABIC - SABIC® PP, SABIC® Vestolen, LNP™ THERMOCOMP™ and more
- RTP Company - ESD C, ESD A, RTP 100, RTP 101 to 109 and more
The plastic database allows you to filter your search based on property (mechanical, electrical etc.), applications, conversion mode and other dimensions for FREE!
How to Produce Polypropylene?
These days, polypropylene is made from polymerization of propene monomer (an unsaturated organic compound - chemical formula C3H6) by:
- Ziegler-Natta polymerization or
- Metallocene catalysis polymerization
 Structure of PP Monomer C3H6 | Ziegler-Natta Polymerization Or Metallocene Catalysis |  Structure of Polypropylene (C3H6)n |
Upon polymerization, PP can form three basic chain structures depending on the position of the methyl groups:
- Atactic (aPP) - Irregular methyl group (CH3) arrangement
- Isotactic (iPP) – Methyl groups (CH3) arranged on one side of the carbon chain
- Syndiotactic (sPP) - Alternating methyl group (CH3) arrangement
 | Polypropylene was first polymerized by German chemist named Karl Rehn and an Italian chemist named Giulio Natta to a crystalline isotactic polymer in 1954. This discovery soon led to a large-scale production of polypropylene starting in 1957 by the Italian firm Montecatini. Syndiotactic polypropylene was also first synthesized by Natta and his coworkers. |
Types of Polypropylene & their Benefits
Homopolymers and Copolymers are the two major types of polypropylene available in the market.
- Polypropylene Homopolymer is the most widely utilized general-purpose grade. It contains only propylene monomer in a semi-crystalline solid form. Main applications include packaging, textiles, healthcare, pipes, automotive and electrical applications.
- Polypropylene Copolymer family is further divided into random copolymers and block copolymers produced by polymerizing of propene and ethane:
- Polypropylene Random Copolymer is produced by polymerizing together ethene and propene. It features Ethene units, usually up to 6% by mass, incorporated randomly in the polypropylene chains. These polymers are flexible and optically clear making them suitable of applications requiring transparency and for products requiring an excellent appearance.
- While in Polypropylene Block Copolymer, ethene content is larger (between 5 and 15%). It has co-monomer units arranged in regular pattern (or blocks). The regular pattern hence makes thermoplastic tougher and less brittle than the random co-polymer. These polymers are suitable for applications requiring high strength, such as industrial usages.
Polypropylene, Impact Copolymer – Propylene Homopolymer containing a co-mixed Propylene Random Copolymer phase which has an ethylene content of 45-65% is referred to PP impact copolymer. It is useful in parts which require good impact resistance. Impact copolymers are mainly used in packaging, houseware, film, and pipe applications, as well as in the automotive and electrical segments.
Expanded Polypropylene - It is a closed-cell bead foam with ultra-low density. EPP is used to produce three-dimensional polymer foam products. EPP bead foam has higher strength to weight ratio, excellent impact resistance, thermal insulation, and chemical and water resistance. EPP is used in various applications ranging from automobiles to packaging, from construction products to consumer goods and more.
Polypropylene Terpolymer - It is composed by propylene segments joined by monomers ethylene and butane (co-monomer) which appear randomly throughout the polymer chain. PP terpolymer has better transparency than PP homo. Also, the incorporation of co-monomers reduces crystalline uniformity in the polymer making it suitable for sealing film applications.
Polypropylene, High Melt Strength (HMS PP)– It is a long chain branched material, which combines both high melt strength and extensibility in the melt phase. PP HMS grades have a wide mechanical property range, high heat stability, good chemical resistance. HMS PP is widely used to produce soft, low density foams for food packaging applications as well as used in automotive and construction industries.
PP Homopolymer vs. Copolymer – How to decide between the two?
| PP Homopolymer | PP Copolymer |
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This is because of their extensively shared properties. As a result, the choice between these two materials is often made based on non-technical criteria.
Interesting Material Properties of Polypropylene
Keeping information about the properties of a thermoplastic beforehand is always beneficial. This helps in selecting the right thermoplastic for an application. It also assists in evaluating if the end use requirement would be fulfilled or not. Here are some key properties and benefits of polypropylene:
- Melting Point of Polypropylene - The melting point of polypropylene occurs at a range.
- Homopolymer: 160 - 165°C
- Copolymer: 135 - 159°C
- Density of Polypropylene - PP is one of the lightest polymers among all commodity plastics. This feature makes it a suitable option for lightweight\weight saving applications.
- Homopolymer: 0.904 – 0.908 g/cm3
- Random Copolymer: 0.904 – 0.908 g/cm3
- Impact Copolymer: 0.898 – 0.900 g/cm3
- Polypropylene Chemical Resistance
- Excellent resistance to diluted and concentrated acids, alcohols and bases
- Good resistance to aldehydes, esters, aliphatic hydrocarbons, ketones
- Limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents
- Flammability: Polypropylene is a highly flammable material
- PP retains mechanical & electrical properties at elevated temperatures, in humid conditions and when submersed in water. It is a water-repellent plastic
- PP has good resistance to environmental stress cracking
- It is sensitive to microbial attacks, such as bacteria and mold
- It exhibits good resistance to steam sterilization
Learn more about all polypropylene properties and their values - ranging from mechanical and electrical to chemical properties; and make the right selection for your application.
How additives help improve PP properties?
Polymer additives like clarifiers, flame retardants, glass fibers, minerals, conductive fillers, lubricants, pigments and many other additives can further improve PP physical and/or mechanical properties. For example:
Search PP Grades Based on Their Reinforcement Method  |
Further, fillers (clays, talc, calcium carbonate…) and reinforcements (glass fiber, carbon fiber…) are added to achieve significant properties related to processing and end use application.
Development & the use of new additives, latest polymerization processes as well as blending solutions significantly increase polypropylene performance. Hence, today PP is less seen as a low cost solution, but much more as a high performance material competing with traditional engineering plastics and sometime event metal (e.g. long glass fiber reinforced PP grades).
Disadvantages of Polypropylene
- Poor resistance to UV, impact and scratches
- Embrittles below -20°C
- Low upper service temperature, 90-120°C
- Attacked by highly oxidizing acids, swell rapidly in chlorinated solvents and aromatics
- Heat-aging stability is adversely affected by contact with metals
- Post molding dimensional changes due to crystallinity effects – this can be solved with nucleating agents » Watch video
- Poor paint adhesion
Key Areas of Applications of Polypropylene
Polypropylene is widely used in various applications due to its good chemical resistance and weldability. Some common uses of polypropylene include:
- Packaging Applications: Good barrier properties, high strength, good surface finish and low cost make Polypropylene ideal for several packaging applications.
- Flexible Packaging: PP films’ excellent optical clarity and low moisture-vapor transmission make it suitable for use in food packaging. Other markets shrink-film overwrap, electronic industry films, graphic arts applications, disposable diaper tabs and closures, etc. PP Film is available either as Cast Film or bi-axially orientated PP (BOPP).
- Rigid Packaging: PP is blow molded to produce crates, bottles, and pots. PP thin walled containers are commonly used for food packaging.
- Consumer Goods: Polypropylene is used in several household products and consumer goods applications including translucent parts, housewares, furniture, appliances, luggage, toys etc.
- Automotive Applications: Due to its low cost, outstanding mechanical properties and moldability, polypropylene is widely used in automotive parts. Main applications include battery cases and trays, bumpers, fender liners, interior trim, instrumental panels and door trims. Other key features of automotive applications of PP include low coefficient of linear thermal expansion and specific gravity, high chemical resistance and good weatherability, processability and impact/stiffness balance.
- Fibers and Fabrics: A large volume of PP utilized in the market segment known as fibers and fabrics. PP fiber is utilized in a host of applications including raffia/slit-film, tape, strapping, bulk continuous filament, staple fibers, spun bond and continuous filament. PP rope and twine are very strong and moisture resistant very suitable for marine applications.
- Medical Applications: Polypropylene is used in various medical applications due to high chemical and bacterial resistance. Also, the medical grade PP exhibits good resistance to steam sterilization. Disposable syringes is the most common medical application of polypropylene. Other applications include medical vials, diagnostic devices, petri dishes, intravenous bottles, specimen bottles, food trays, pans, pill containers, etc.
- Industrial Applications: Polypropylene sheets are widely used in industrial sector to produce acid and chemical tanks, sheets, pipes, Returnable Transport Packaging (RTP), etc. because of its properties like high tensile strength, resistance to high temperatures and corrosion resistance.
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Usefulness of Polypropylene Films
PP film is among the leading materials today used for flexible packaging as well as industrial applications. Two important forms of polypropylene films include:
Cast Polypropylene Film
Cast polypropylene commonly known as CPP and widely known for its versatility.
- Super resistance to tears & puncture
- Greater transparency and better heat resistance at high temperatures.
- Excellent moisture and atmospheric barriers
- High permeability to water vapor
Biaxially Oriented Polypropylene Film
Biaxially oriented polypropylene film (BOPP) is stretched in both transversal and longitudinal directions, producing molecular chain orientation in two directions.
- Orientation increases tensile strength and stiffness
- Good puncture and flex crack resistance over wide range of temperatures
- Have excellent gloss and high transparency can be glossy, clear, opaque, matte or metalized
- Efficient barrier against oxygen and moisture
PP vs. PE – Selecting the Suitable Polymer
Though Polyethylene and Polypropylene are similar in physical properties but here are key points to consider to select the polymer suitable to your needs.
| Polypropylene | Polyethylene |
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| » View all PP Commercial Grades | » View all PE Commercial Grades |
Polypropylene Processing Conditions
Polypropylene can be processed virtually by all processing methods. The most typical processing methods include: Injection Molding, Extrusion, Blow Molding and General-Purpose Extrusion.
- Injection Molding
- Melt temperature: 200-300°C
- Mold temperature: 10-80°C
- Drying is not necessary if stored properly
- High mold temperature will improve brilliance and appearance of the part
- Mold shrinkage lies between 1.5 and 3%, depending on processing conditions, rheology of the polymer and thickness of the final piece
- Extrusion (tubes, blow and cast films, cables, etc.)
- Melt temperature: 200-300°C
- Compression Ratio: 3:1
- Cylinder Temperatures: 180-205°C
- Pre-Drying: No, 3 hours at 105-110°C (221-230°F) for regrind
- Blow molding
- Compression molding
- Rotational molding
- Injection blow molding
- Extrusion Blow Molding
- Injection stretch blow molding
- General-Purpose Extrusion
Expanded Polypropylene (EPP) may be molded in a specialized process. Being an ideal material for injection molding process, it is majorly used for batch and continuous production.
3D Printing with Polypropylene
As a tough, fatigue resistant and durable polymer, PP is ideal for low strength applications. Due to its semi-crystalline structure and heavy warping, it is currently difficult to use polypropylene for 3D Printing processes.
Today, several manufacturers have optimized PP properties or even created blends with improved toughness making it suitable for 3D Printing applications. Hence, it is recommended to thoroughly refer to documentation provided by supplier for printing temperature, printing bed, etc., while 3D Printing with polypropylene...View all PP Grades Suitable for 3D Printing
Polypropylene is suitable for:
- Complex models
- Prototypes
- Small series of components, and
- Functional models
(Credit: FormFutura)
Is PP toxic? How to recycle PP?
All plastics have a ‘Resin Identification Code/ Plastic Recycling Code’ based on the type of resin used. PP’s resin identification code is 5.
PP is 100% recyclable. Automobile battery cases, signal lights, battery cables, brooms, brushes, ice scrapers, etc., are few examples which can be made from recycled polypropylene (rPP).
The PP recycling process mainly includes melting of waste plastic to 250°C to get rid of contaminants followed by removal of residual molecules under vacuum and solidification at nearly 140°C. This recycled PP can be blended with virgin PP at a rate upto 50%. The main challenge in PP recycling is related to its amount consumed – currently nearly 1% PP bottles are recycled as compared to 98% recycling rate of PET & HDPE bottles together.
The use of PP is considered safe because it does not have any remarkable effect from an occupational health and safety point of view, in terms of chemical toxicity.
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